This invention relates to electromagnetic heating and in particular to a method and apparatus for sealing by induction heating in environments where the use of known induction heaters is limited.
Induction heating is well known in the art. When a current passes through a conductive coil, a magnetic flux is produced inside and around the coil. When a mass of conducting metal is moved in this magnetic field or is subjected to a changing flux, induced currents called eddy currents circulate in the mass. In most instances, eddy currents are wasted energy, but in induction heating they are the actual producers of heat.
When welding thermoplastics, one places conducting metal either on top of a piece to be heated or between the two pieces to be joined. Thermoplastics are transparent to magnetic flux, so an induction heater can direct flux through the plastic to the metal. The heat produced in the conducting metal softens and joins the plastics.
Many types of induction heaters are presently in use. Their use is limited, however, by several disadvantages. Induction heating presently involves passing a high-power current, often of several kilowatts, through an inducting coil to effect high heat in the conducting metal. The coil suffers self-induced power losses which are manifested as heat in the coil. If low power currents are passed through the coil, as in the invention of U.S. Pat. No. 3,864,186, heat induced in the conducting metal is low.
Air or fluid cooling has sufficed to keep high-power systems from overheating, as in the heaters of U.S. Pat. Nos. 4,120,712 and 3,738,892, but in a vacuum environment such as outer space, cooling systems are impractical, because energy is not readily available for their operation. Without a cooling system, an induction heater remains cool only if the coil is large enough to accommodate what power passes through it. A coil of a size able to accommodate the kilowatts necessary for induction of intense heat is prohibitively large.
Induction heaters now in use are also limited in their efficiency. Generally, magnetic flux travels to the material to be heated in one of two ways. Either the coil is set atop the piece to be heated, or such a piece is placed within the coil. The latter method maximizes efficiency, but limits the size of materials which may be joined. Use of the former method results in losses of magnetic flux all around the coil.
In fabrication of large items such as space structures, a portable induction heater which needs no external cooling is desirable for sealing seams and joining components. A portable heater ideally uses little power, which power may be readily obtained, and must be able to be controlled by easily miniaturized components.
Accordingly, it is an object of this invention to provide a method and apparatus for sealing thermoplastics and organic matrix composites, which can be used in outer space, in the environment of earth, or in motionless surroundings.
It is another object of this invention to provide such an apparatus, wherein an air or fluid cooling system need not be used.
It is another object of this invention to provide an induction heating apparatus which makes use of essentially all the flux generated by an induction coil.
It is still another object of this invention to provide such an apparatus, wherein the flux produced may be transferred efficiently to a body outside of the induction coil.
It is another object of this invention to provide an induction heating apparatus which is portable and uses low input power to accomplish high-temperature melting and joining of thermoplastics.
It is yet another object of this invention to provide such an apparatus wherein power may be supplied by a small area of solar paneling.
It is another object of this invention to provide a method of joining metals using induction heating techniques.